A very slight examination of one of the Sloths showed that it had no front teeth, that is to say, neither incisors nor true canine teeth, and that the hinder teeth--the false and true molars--were not like those of any other mammal. The back teeth, few in number, have since been ascertained to be exceptionally simple in their structure, and evidently the masticating process is very simple. But when it was noticed that the Sloth fed upon leaves and young twigs, the absence of the necessity for more elaborate teeth was acknowledged. Then it was observed that they had very long arms, or rather fore limbs, for the fore-arm bones and the humerus are all unusually long, and also that they had great power of movement. Moreover, it was seen that the fingers were reduced to three in number in some kinds, and to two in others, and that they were furnished with long and strong claws, which did not interfere with a great amount of mobility in the wrist. The length of limb, the mobility of the wrist, and the great claws, enable the Sloth to bring the leaves to its mouth, to hang on, and to walk, as it were, beneath the branches. An examination of the hind limbs showed that they were shorter than the others, and always furnished, in all kinds of Sloths, with three great claws. But the ankle seemed to be turned in, as if there was a state of “club-foot.” This condition would enable the toes to clasp a bough without effort, but it would prevent the sole from being placed flat on the ground. As the knowledge of the anatomy of these constant tree-livers progressed, other modifications of structure, equally important in relation to the peculiar arboreal life and food, were gradually discovered. For instance, a remarkable flexibility of the neck, produced by the peculiar arrangement of the vertebræ; a _rete mirabile_, to a certain extent, in the limbs, resembling somewhat that in the Lemurs (Vol. I., pages 213, 245), and a complicated stomach suited for the digestion of leaves, and foreshadowing that of the Ruminants. Two different kinds of Sloths were described in the first instance, and subsequently, several others. The first kinds known were the Ai, a Sloth with three claws on the fore limb, and the Two-toed Sloth, with two claws on the fore limb. The Ai was called _Bradypus tridactylus_, and the other the Unau, or _Bradypus didactylus_, names which have been changed somewhat, as will be seen further on. Sloths are caught without much difficulty, and their habits, in captivity, have been observed in South America, and also after their removal to Europe. Waterton writes[56] on the subject:--